build own filter callback function

Question

If I call the filter function, I get this array returned [ 1, , 3, , 5 ]. From where come the additional commas? I don't understand this effect. Can somebody explain it to me? The array should be that: [ 1, 3, 5 ].

class List {
    constructor(values = []) {
        this._list = values;
    }

    
	filter(func) {
		let newList = new Array();
		let indexList = 0;
		let indexNewList = 0;

		while (this._list[indexList] != undefined) {
			if (func(this._list[indexList]) === true) {
				newList[indexNewList] = this._list[indexList];
				indexNewList++;
			}
			indexList++;
		}
		this._list = newList;
		return this;
	}
  
  get values() { return this._list }
}

var isOdd = function (x) {
    return x % 2 === 1;
};

var list = new List([1, 2, 3, 4, 5]);
console.log(list.filter(isOdd).values);

Shadowfacts · Accepted Answer · 2018-03-19 01:21:16Z

1

If an item in the list matches the filter, you're inserting it into the new list at the index of the item in the original list. You want to simply be appending the item to the new list.

Use another variable to keep track of what index the element should be inserted into the new list at:

let newList = new Array();
let indexList = 0;
let newIndex = 0;

while (this._list[indexList] != undefined) {
    if (func(this._list[indexList]) === true) {
        newList[newIndex] = this._list[indexList];
        newIndex++;
    }
    indexList++;
}

The newIndex variable will only be incremented when an item has been inserted into newList, instead of being incremented with every iteration of the loop.

edited Mar 19, 2018 at 1:21

answered Mar 19, 2018 at 1:05

Shadowfacts

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4 Comments

skinnyBug Over a year ago

no i cant use push, i have to program it without using existing functions. But you got me to the right solution. I didn't recognize that i need a second index counter. Thanks! I changed it above now it is working!

Shadowfacts Over a year ago

In that case, set the element at the index that is the length of the array. I've updated my answer.

skinnyBug Over a year ago

But length is also an existing function

Shadowfacts Over a year ago

Use another variable to keep track of the index to insert into the new list at. I've updated my answer.

Ele · Accepted Answer · 2018-03-19 01:22:18Z

The problem is the increment of the variable index, that increment is creating empty/undefined elements.

For example:

Array = [1];
index = 1
callback returns false
The index is incremented by 1 -> index =2`
Next iteration callback returns true
A new element is added to Array at position 2 ->
Array = [1, undefined, 3].

Use a separated index for the newArray.

class List {
  constructor(values = []) {
    this._list = values;
  }

  filter(func) {
    let newList = new Array();
    let index = 0;
    let newListIndex = 0;

    while (this._list[index] != undefined) {
      if (func(this._list[index]) === true) {
        newList[newListIndex++] = (this._list[index]);
      }
      index++;
    }
    this._list = newList;
    return this;
  }

  get values() {
    return this._list
  }
}

var isOdd = function(x) {
  return x % 2 === 1;
};

var list = new List([1, 2, 3, 4, 5]);
console.log(list.filter(isOdd));

Shanon Jackson · Accepted Answer · 2018-03-19 01:33:10Z

I would suggest sticking with a functional-programming style definition of array#filter since that is the paradigm it originates from.

This is the fully immutable version of List#filter where you get a new instance of List back and the underlying array never goes through any form of mutation.

class List {
  constructor(values = []) {
    this._list = values;
  }

  filter(func) {
    var reduce = function(values, accum) {
      if(values.length === 0) return accum;
      if(func(values[0])) return reduce(values.slice(1), accum.concat(values[0]));
      else return reduce(values.slice(1), accum)
    }
    return new List(reduce(this._list, []))
  }

  get values() {
    return this._list
  }
}

var isOdd = function(x) {
  return x % 2 === 1;
};

var list = new List([1, 2, 3, 4, 5]);
console.log(list.filter(isOdd));

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